Potential conflict of interest: The study was funded by an unrestricted grant from GE Healthcare (formerly Amersham Health) to PBE consulting, Verona, Italy (Berto and Lopatriello). At the time of the study Michael Chambers was an employee of GE Healthcare. Lieven Annemans received an unrestricted grant from GE healthcare for the development of the original model. Angelo Antonini and Filippo Tamma have received consultancy fees from GE Healthcare.